Multiple battery charger with a power source control

ABSTRACT

A battery charging system is disclosed that includes an efficient switch mode power supply, multiple linear current limiters, and feedback means to allow the switch mode power supply to operate at the minimum voltage necessary to operate a power load and charge batteries. Furthermore, the switch mode power supply is capable of producing the maximum power required by the system, such as when the battery charger is used in conjunction with operation of a electronic device with peak load demands such as when a hard disk is accessed in a portable computer. Two control mechanisms are found in the battery charging system. The first mechanism is an input used to control the switch mode power supply output voltage from an external source. In one embodiment, this is done by referencing a first voltage to that internal to the switch mode power supply. The second mechanism is to limit the current within the switch mode power supply not controlled by an external source. This current limiting feature is linear and is set at an absolute limit point. Each limiter supplies charge current to a battery, which charge current is monitored and compared to a reference voltage with the resulting error voltage used to control the output of a series pass controller, such as a transistor.

BACKGROUND OF THE INVENTION

The present invention is directed towards an efficient battery charger,and more specifically, to an efficient multiple battery charger havingan improved power controller that senses the power demand on eachbattery and provides the minimum output voltage required to charge all,including the battery with greatest demand.

In battery powered portable electronic systems the portable power supplyis usually provided by either disposable batteries, replaceablebatteries, such as Nickel Cadmium (NiCd), or other rechargeablebatteries. The disposable batteries are well known, but are notrechargeable. Since many users desire to use their electronic devicewithout regard to whether an electrical outlet is available to power thedevice, such consumers typically rely on rechargeable batteries to powerthe device. One such type of battery is the rechargeable battery, whichtypically requires a dedicated battery charger. Another type of batteryis a battery pack specifically designed for the portable electronicdevice. In this type of device, the manufacturer of the device allowsthe battery pack to be replaceable so that a second battery source canbe carried to replace the first pack in the event that the first batterypack has its charge depleted. The next type of batteries are thoseintended to be permanently installed in the device. Once the batteriesare drained of their energy, either they must be recharged or the devicemust be powered by an alternative power source before the device canresume operation.

In devices that use either separate battery power packs or internalbatteries, several batteries are charged during the same rechargingsession. Most frequently, each battery is at a different voltage levelfrom the others in the pack. This leads to an inefficient chargingscheme when all batteries are being recharged simultaneously.Alternatively, some systems attempt to recharge each battery separatelyfrom the others, but do so in a manner that is highly inefficient inthat the charging load does not adjust as the voltage charge on eachbattery reaches a full charge state. This power loss is usually in theform of heat, which can be damaging to the electronic device if any ofthe fail safe mechanisms in the device fail under certain circumstances.

Accordingly, what is needed is a battery charger with a power controllerthat operates at the minimum voltage possible in order to reduce powerloss and heat. Furthermore, the power charger should be able to respondto peak loads when used in combination with the powering of theelectronic device while charging the batteries to operate the devicewithout interfering with system performance.

SUMMARY OF THE INVENTION

According to the invention, a battery charging system is disclosed usinga power supply, such as a switch mode power supply, and multiple linearcurrent limiters. The limiters use feedback means associated with thebattery charger to allow the switch mode power supply to operate at theminimum voltage necessary to operate a power load and charge batteries.This system also allows the switch mode power supply to produce maximumpower required by the system. Such occurs when the battery charger isused during the same operation of an electronic device with peak loaddemands, such as when a hard disk is accessed in a portable computer.

The battery system further includes two control mechanisms forincreasing operating efficiency. The first mechanism is an input forcontrolling the switch mode power supply output voltage from an externalsource. In one embodiment, this is done by referencing a first voltageto the voltage internal to the switch mode power supply. The secondmechanism limits the current within the switch mode power supply notcontrolled by an external source. This current limiting feature islinear and is set at an absolute limit point. Each limiter suppliescharge current to a battery. Each charge current is monitored andcompared to a reference voltage with the resulting error voltage used tocontrol the output of a series pass controller, such as a transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a battery charger according to the presentinvention, which is able to charge multiple batteries at the minimumvoltage possible; and

FIGS. 2A, 2B and 2C are schematic diagrams illustrating the specificembodiment of the battery charger in FIG. 1.

DETAIL DESCRIPTION OF SPECIFIC EMBODIMENTS

A battery charger 10 with an efficient power controller 12 is disclosedin FIG. 1 and described below. Battery charger 10 includes a powersource 14, typically an alternating current voltage source such as, forexample, the 110 AC voltage from any electrical outlet common to mostbuildings. The power source 14 modifies and rectifies the voltage andthen steps it down to a level useable by the battery charger 10 and,subsequently, the load 16 to be powered. These techniques are well knownin the art and are left to the skilled artisan for implementation. Thepower source may also be the type used and described in, for example,commonly assigned U.S. patent application Ser. No. 08/080,384, titledUniversal Power Converter, filed herewith and herein incorporated byreference for all purposes. The power source 14 is further connected toa noise filter 18, which is used to filter the power supplied to thebattery charger 10 and load 16.

The power source is further connected to various current controlcircuits 20, 22 and 24, which are used to regulate the current, thuslimiting the voltage applied to their respective batteries, 20B, 22B and24B. Each of the current control circuits 20, 22 and 24 are furtherconnected to their respective battery 20B, 22B, and 24B. According tothe present invention, the number of control circuits is determined bythe number of battery packs, which can be any number N. One currentcontrol circuit is provided for each battery pack. Each battery 20B, 22Band 24B is further connected to the load 16, such as, for example, apersonal portable computer, a cellular phone, a portable stereo, or anyother like electronic device.

Furthermore, the batteries may use non-dissipative charges equalizationcontrols as, for example, described in commonly assigned U.S. patentapplication Ser. No. 08/080,898, titled Non-Dissipative Battery ChargerEqualizer, filed herewith and incorporated by reference for allpurposes.

Finally, the power source 14 is further connected to a voltage reference26, which is used to control the current in each current control circuit20, 22 and 24 and subsequently to control the voltage applied to eachbattery as charged.

The power supply 14 intended for use in the battery charger 10 is basedon a switch mode power supply and is used in conjunction with multiplelinear current limiters, which are implemented in the current controlcircuits 20, 22 and 24. Feedback amplifier 28 is used in conjunctionwith the current limiters to operate the switch mode power supply insuch a manner as to minimize the voltage output for charging thebatteries or operating the system. The switch mode power supply iscapable of producing a maximum power required by the system. This systemuses two control mechanisms. The first is an input used to control theswitch mode power supply output voltage from an external source. This istypically in the form of a voltage that is compared with a referenceinternal to the switch mode power supply. The second mechanism iscurrent limiting within the switch mode supply not controlled by anexternal source. The current limiting is linear and is set at anabsolute limit point. Other types of current limiting are not intendedto be used, which types include hold back or burp mode limiting.Importantly, the switch mode power supply must operate reliably ineither the voltage or current modes. This is accomplished by causing theswitch mode power supply to limit current in a linear fashion.

Each current limiter module then supplies the charge current to itsrespective battery. The charge current is monitored and compared to areference and the resulting error voltage is used to control the outputof a series pass element, which obtains its voltage from the switch modepower supply. Each battery and current sense device may have in parallela computing device or other electrically powered component and,therefore, the series pass element must be capable of supplying thetotal of component and battery charging current.

Several current limiting circuits, or current control circuits may bedriven in parallel from a single switch mode power supply. Since any ofthe batteries can be charging at a voltage different than the others atany given point in time, the switch mode power supply needs to developsufficient voltage to allow charging the battery with the highestvoltage. To reduce power loss and heat in the series pass elements, andto keep such losses at a minimum, the switch mode power supply output isnot allowed to be greater than the highest voltage required. Eachcurrent source control circuit then has a secondary output that iscombined with the other signals in such a way that the one with thegreatest signal causing that pass element to be saturated is fed back tothe switch mode power supply, thereby causing the power supply to reduceits output to maintain the correct charge current for that battery. Bythis means, there is always one of several series pass elements that isfully saturated, insuring that the output of the switch mode powersupply will be no greater than the minimum required.

Based on this arrangement, during moments of peak loading, such asspinning up a hard disk, a CD-ROM drive, or any other like peak loadoperation, the switch mode power supply is not required to continue fullrapid charging on all batteries and reduces its output below that whichwill sustain rapid charge in all batteries, which action is accomplishedby the internal current limiter. This diverts the current necessary tosupply the power to the demand load momentarily.

A preferred embodiment of the present invention is illustrated in FIG.2, which includes FIGS. 2A, 2B and 2C. In FIGS. 2A and 2B the batterycharger 110 is connected to a power source, or power supply (not shown),which offers two levels of power for use in the battery charger 110. Thefirst level of power comes in on power main line 128. The second comesin on power auxiliary line 130. The main power line 128 supplies aroundthree (3) Amperes (A) of current, which the auxiliary power line 130supplies 168 mA. The main power line 128 services the current controlcircuits 122, 123, 124 for recharging the batteries (not shown). Theauxiliary power line 130 provides operating current for the currentcontrol circuitry and substantially follows the main input voltagelevel.

A voltage reference element 132, which is powered by the auxiliary powerline, provides a stable 2.5 volt reference voltage at the cathode of theSchottky Diode U5. This voltage is divided by resistor pairs R4-R5,R8-R9, and R12-R13, for current control circuits 122,123, and 124,respectively. The reduced referenced voltage from each voltage dividerpair R4-R5, R8-R9, and R12-R13 is then presented to the inverting inputsof amplifiers U1, U2, and U3, respectively. Divider resistors R5, R9,and R13 are grounded remotely to the respective battery pack negativeterminals, to reduce the effects of current on the power ground circuit.Each current control circuit 122, 123, and 124 is connected to arespective battery pack (See FIG. 2C) wherein a current output line(OUT) 136A, 136B, and 136C, a current sense (I.SENS) line 138A, 138B,and 138C, a control sense (C.SENS) line 134A, 134B, and 134C, and aground (GND) line 140A, 140B, and 140C are coupled to the battery pack.

In FIG. 2C, the battery 220 in the battery pack 120 is connected betweenthe OUT line 136 and the current sense lines, while a regulatingresistor 222 is connected in series with the battery 220 and between theI. SENS. line 138 and the C. SENS. line 134. Thus, the battery 220 andresistor 222 are connected between the OUTput line 136 and the I. SENS.and GND. lines 138 and 140, respectively. A resistive load RLOAD, suchas a microprocessor of a portable computing device, may be connected inparallel to the battery pack 120.

As illustrated in FIGS. 2A and 2B the C. SENS. lines 134A, 134B, and134C are connected to noninverting inputs of each of the current senseamplifiers U1, U2, and U3, respectively. Current through a given batteryis then represented as a voltage at the noninverting input of thecorresponding current sense amplifier U1, U2, or U3. The sensed voltageis then compared with the divided reference voltage to cause theamplifier output to respond in a manner that will control the currentthrough the battery.

Current to the batteries is supplied through pass transistors Q3, Q5,and Q7, which are P-channel MOS field effect transistors (FET), and areconnected to the main power line 128 on the source side of thetransistor and to the battery on the drain side of the transistor, withthe gate being coupled to the output of the current sense amplifiers U1,U2, and U3, respectively. Charging current is controlled by varying thegate voltage to the pass transistor Q3, Q5, and Q7 from the outputs ofthe current sense amplifiers U1, U2, and U3, respectively, throughcurrent shut off transistors Q2, Q4, and Q6, respectively. Each currentshut off transistor Q2, Q4, and Q6, respectively, is turned off duringthe absence of input power to prevent the batteries from dischargingthrough resistors R6, R10, and R14, respectively, which resistors arecurrent pull up resistors between the respective gate and source of eachpass transistor Q3, Q5, and Q7.

Since the pass transistors Q3, Q5, and Q7 are P-channel MOSFETs, theyincrease conduction as the gate voltage is made to go increasinglynegative with respect to the FET source. As the proper charging currentis reached, the output of each current sense amplifier U1, U2, and U3then increases to provide a gate voltage that maintains the desiredcurrent.

A buffer amplifier U4 provides an output voltage to control the maininput voltage for each current control circuit 122,123, and 124. Theconnection to amplifier U4 is the power to amplifiers U1-U4. As theoutput voltage decreases, the main supply output voltage is caused toincrease. The output of buffer amplifier U4 is controlled by the lowestsignal of all the output signals from current sense amplifiers U1, U2,and U3 through their respective diodes CR1, CR2, and CR3. The input tobuffer amplifier U4 is biased by resistor R15.

During charging operation, the battery having the highest initial chargewill demand the greatest charging voltage. This demand causes the outputof its respective error amplifier to decrease in an effort to increasethe output of the corresponding pass transistor Q3, Q5, or Q7. Thiscauses a response at the output of buffer amplifier U4 to force the mainpower supply line 128 to provide the required voltage to the input ofthe pass transistor Q3, Q5, or Q7 associated with the battery having thegreatest charging voltage demand. In this manner, the pass transistorassociated with the battery having the greatest charging voltage demandbecomes fully saturated. Via this means, the charging voltage suppliedto the controller is held at the minimum voltage required to charge allbatteries, thereby reducing the power loss in the main supply and thecharge control pass transistors to a minimum. Each current controlcircuit 122, 123, and 124 further includes a stabilizing or integratingcapacitor C3, C4, and C5, respectively. Furthermore, resistor R15 servesas a bias resistor for each control diode CR1, CR2, and CR3. Inaddition, resistor R16 provides a 2.5 volt reference to ground atvoltage reference U5. Resistor R17 is connected between the auxiliarypower line and the voltage reference U5 to provided operating currentfor the 2.5 volts reference.

Because of the large power handling capabilities of pass transistors Q3,Q5, and Q7, each is mounted on a heat sink to help dissipate the heatgenerated inside the transistor. It should be noted that the circuitryused in this battery charger control circuit may be implemented in asingle integrated chip. Such a chip may be mounted on a single heatsink. On the other hand, the pass transistors may be integrated togetheron the same chip but separate from the other elements in each currentcontrol circuit. This allows the pass transistors to be mounted to thesame heat sink.

The power dissipated by the series pass elements (charger) depends oncharging/load current multiplied by the voltage across the element.Since current is constant (except for trickle charge), power loss isproportional to series pass voltage. Since minimum series pass voltageis maintained, the worst case power loss quickly becomes quite small asbattery voltages equalize. As batteries go into trickle charge, powerloss becomes negligible. As batteries become similar in voltage and aseach battery reaches a full charge, power dissipation is held at aminimum and is eventually reduced to only that necessary to charge theremaining batteries. Once the batteries are fully charged, a tricklecharge of only 45 milliwatts is required to maintain the batteries at afull charge state while the power or battery charger is in operation. Intypical battery charging systems, the power dissipation does not reducegradually in a manner that tracks the power requirements to charge thebatteries efficiently, rather, the power dissipation is either at fulllevel, for example, 11 watts, or at a trickle charge state of, forexample, 45 milliwatts, with no power variation in between. Accordingly,it is apparent that there is a large amount of power loss associatedwith the prior battery charging systems that are either on full power orat a trickle charge without any power tracking capabilities as thebatteries go from reduced power consumption a full depletion state andfull charge state.

Since the auxiliary power line is used to provide operating current tothe current control circuits, a noise filter 118 is provided to filterthe voltage before reaching the current control circuits 122, 123, 124.The noise filter 118 includes a pair of capacitors C1 and C2, which areoptimized to provide the best noise filtering capabilities. These twocapacitors are connected together in parallel between the auxiliarypower line 130 and ground 140. A transistor Q1 is connected between theauxiliary power line 130 and the power enable line 142 of the powersupply, which line 142 is used to turn on monitoring circuits in thebattery packs when power is available.

A novel battery charging system for charging multiple batteries has beendescribed. It will be understood by those skilled in the art thatvariations and modifications can be effected within the spirit and scopeof the invention as described above and as defined in the followingclaims.

What is claimed is:
 1. A battery charging apparatus which continuously.charges multiple batteries during a charging cycle, comprising:a powersupply which provides charging current to all batteries simultaneously;a plurality of linear current limiters coupled to said power supply andeach linear current limiter further connected to a respective battery tocontinuously provide charging current to said respective battery duringsaid charging cycle, wherein each current limiter includes means tocontrol the amount of charging current supplied to said respectivebattery; and means, coupled to said power supply and to each battery,and to each of said plurality of linear current limiters, for providingfeedback information indicative of a voltage level of said respectivebattery to each of said plurality of current limiters, wherein each ofsaid plurality of linear current limiters reduces the voltage dropacross itself in response to said feedback information to maintainsubstantially constant current through said respective battery duringsaid charging cycle.
 2. The apparatus of claim 1 wherein said powersupply is a switch mode power supply.
 3. The apparatus of claim 1wherein each of said plurality of linear current limiters comprises aseries pass transistor coupled between said power supply and saidbattery for controlling the voltage applied to said battery.
 4. Abattery charging apparatus which continuously charges at least first andsecond batteries during a charging cycle, said apparatus comprising:apower supply which provides charging current to said first and secondbatteries simultaneously; and at least first and second currentlimiters, wherein each current limiter is connected between said powersupply and a respective one of said at least first and second batteriesto continuously supply charging current to said battery connectedthereto during said charging cycle, and wherein each current limitersenses a voltage proportional to an actual current level through saidbattery connected thereto, each current limiter comparing said sensedvoltage with a reference voltage to obtain a feedback signal used toadjust the voltage drop across itself so as to maintain substantiallyconstant charging current through said battery connected thereto.
 5. Thebattery charging apparatus of claim 4, wherein each current limiterincludes a pass transistor having a source connected to said powersupply, a drain connected to said respective battery, and a gateconnected to said feedback signal such that the amount of voltage dropacross each current limiter is varied based upon the level of saidfeedback signal applied to said gate.
 6. The battery charging apparatusof claim 4, wherein the feedback signal from each current limiter isconnected to a feedback amplifier such that the feedback amplifiergenerates a voltage control output signal based upon a greatest actualcurrent level from a battery having a greatest voltage level, whereinsaid power supply receives said voltage control output signal andadjusts the voltage output therefrom in accordance with said voltagecontrol output signal to a level required to charge the battery with thegreatest voltage.
 7. A battery charger which continuously charges aplurality of batteries during a charging cycle comprising:a power supplywhich provides a voltage output to all batteries simultaneously; aplurality of current control circuits, wherein each current controlcircuit is connected between said power supply and a respective one ofsaid batteries to continuously supply charging current to saidrespective battery during said charging cycle, each of said currentcontrol circuits sensing a voltage proportional to an actual currentlevel through said respective battery and comparing said sensed voltagewith a reference voltage to obtain a feedback signal indicative of thevoltage level of said respective battery; and a feedback circuit whichreceives said feedback signals from said current control circuits andgenerates a voltage control output signal based upon a lowest levelfeedback signal, wherein said power supply receives said voltage controloutput signal and adjusts the voltage output therefrom to a levelrequired to charge the battery with the greatest voltage level.
 8. Thebattery charging apparatus of claim 7, wherein each current controlcircuit includes a pass transistor having a source connected to saidpower supply, a drain connected to said respective battery, and a gateconnected to said feedback signal such that the amount of voltage dropacross each current control circuit is varied based upon the level ofsaid feedback signal applied to said gate.